Hydrogenation Effect of Amorphous Silicon Thin Film Transistors by Atmospheric Pressure CVD
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HYDROGENATION EFFECT OF AMORPHOUS SILICON THIN FILM TRANSISTORS BY ATMOSPHERIC PRESSURE CVD
Byung Chul AHN, Jeong Hyun KIM and Dong Gil KIM*, Byeong Yeon MOON, 5 Kwang Nam KIM, Chan Woo LEE, and Jin JANG * * Anyang Research Lab., GoldStar Co. Ltd., Anyang-shi 430-080, Korea * Kyung Hee University, Dongdaemoon-ku, Seoul 130-701, Korea ABSTRACT The hydrogenation effect was studied in the fabrication of amorphous silicon thin film transistor using APCVD technique. The inverse staggered type a-Si TFTs were fabricated with the deposited a-Si and SiOz films by the atmospheric pressure (AP) CVD. The field effect mobility of the fabricated a-Si TFT is 0.79 cm 2 /Vs and threshold voltage is 5.4V after post hydrogenation. These results can be applied to make low cost a-Si TFT array using an in-line APCVD system. INTRODUCTION It is well known that the hydrogenated amorphous silicon(a-Si:H) is the most suitable material for the thin film transistor (TFT) in the flat panel displays. The plasma enhanced (PE) CVD which is used to make the thin film deposition of a-Si:H and SiNx has serious problems such as low throughput, low film quality, pinholes in the deposited insulator film due to the dust generated [1]. These drawbacks of the PECVD method make it hard to cut down the production cost of TFT-LCD panels. In order to overcome these problems, the APCVD technique was proposed by Ahn et al. [2]. The deposition rate of a-Si increases with the partial pressure of disilane and with the total gas pressure, so that high deposition rate can be obtained by thermal CVD at atmospheric pressure [2,31. The deposition of gate insulator by plasma CVD is time consuming process because the thickness of this layer should be higher than 3500. to remove the dielectric breakdown by plasma ions. In this work we used APCVD SiO2 film, which can be deposited with high rate without plasma. Therefore, APCVD SiO2 is effective to enhance the production throughput of a-Si TFT arrays. effective In order to apply the a-Si by APCVD to the fabrication of TFTs, hydrogenation of a-Si is necessary [4-71. In this work the inverted staggered TFTs were fabricated using the APCVD a-Si and SiOz films. In-situ and ex-situ hydrogenations were carried out to get the high performance of TFT. And the a-Si/SiO2 interface was treated with N2 plasma to improve the interface states. EXPERIMENTAL A schematic diagram of the APCVD system used in this work is shown in Figure 1. A quartz tube 6.8 cm in inner diameter and 43 cm in length was evacuated by a turbo molecular pump to a base pressure of about 10-6 Torr. Glass plates were placed on silicon wafer susceptor and heated up by halogen lamps placed at the lower half of the quartz tube. The outside of the quartz tube, surrounded by the tungsten-halogen lamps, was cooled down by the cooling air to prevent the deposition on the inner side of the quartz tube, so that this system is a cold wall type [8]. A disilane/helium mixture gas and hydrogen were used for the a-Si deposition and the post hydrogenation, respectively [9]. A RF co
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